Preserve final-bar sell signal until next market open #6

Problem

When an exit rule fires on the final input bar, the core returns .exit but also synthesizes a completed trade at that bar's close. This contradicts next-market-open execution semantics.

Acceptance criteria

• Final-bar exit remains pending and emits no completed trade for that position.
• Adding the next bar completes the trade at the next bar's open.
• Existing open-position finalization behavior without a pending exit remains unchanged.
• Targeted and full Swift tests pass.